In recent years, resins produced from bisphenols, for example, epoxy resin, polyester resin, polycarbonate resin and polyester carbonate resin are widely used as materials having heat resistance, transparency, impact resistance and a high refractive index. Especially resins comprising a fluorene derivative such as 9,9-bis(4-(2-hydroxyethoxy)phenyl)fluorene are promising materials having high transparency and a high refractive index and are expected to be used as optical materials for auto head lamp lenses, CD's, CD-ROM's, pick-up lenses, Fresnel lenses, fθ lenses for laser printers, camera lenses and projection lenses for rear projection TVs.
Out of these, resins comprising an aromatic dicarboxylic acid and a fluorene derivative and resins comprising an aromatic diol and a fluorene derivative are proposed as materials for optical lenses having a high refractive index and little birefringence (Patent Documents 1 and 2). However, since these resins are colored during melt polymerization, products which are excellent in hue may not be obtained.
To solve this problem, there are reports concerning the hue of a resin comprising a fluorene derivative (Patent Documents 3 and 4). In these reports, the coloration of the resin can be suppressed by reducing the content of the residual sulfur at the time of producing a fluorene derivative and the contents of sulfur and diethylene glycol in the resin.
However, since there is a case where the resin is colored even in the above method, there is still room for the improvement of the resin. Therefore, these problems must be solved in order to widely use the resin as an optical material.
(Patent Document 1) U.S. Pat. No. 2,843,215
(Patent Document 2) JP-A 10-101786
(Patent Document 3) JP-A 2008-111047
(Patent Document 4) JP-A 2009-185299